U.S. Pat. No. 5,007,046 issued Apr. 9, 1991 in the name of Richard H. Erving et al for Computer Controlled Adaptive Speakerphone and assigned to the assignee hereof and incorporated herein by reference and made a part hereof (the "'046 patent") discloses a speakerphone adapted to be in an idle state, a transmit state or a receive state, and to switch from any one of such states to either of the other two states. The same speakerphone is also disclosed in U.S. Pat. No. 4,959,857, U.S. Pat. Nos. 4,901,346 and 4,887,288, each assigned to the assignee hereof, but it will be discussed herein with reference to its disclosure in the '046 patent.
The speakerphone disclosed in that patent, under the control of, for example, a computer, measures the energy of the incoming transmit and receive signals and also develops information about the signal and noise levels for self calibration and efficient operation. This information is obtained for the computer illustratively by pre-processing analog circuitry and an analog-to-digital converter. The analog circuitry converts the incoming transmit and receive signals into a signal that tracks the envelope of the audio. This envelope information is then amplified by a logarithmic amplifier which greatly expands the dynamic operating range of the speakerphone. The resulting analog signals are passed to the analog-to-digital converter which periodically presents the computer with digital information corresponding to the logarithm of the amplitude of the envelope of the signals.
This digital information is used by the computer to develop several different audio signal averages. A transmit signal average and a receive signal average are developed by averaging samples of these signals in a manner that recognizes peaks in the applied signals. Since speech tends to have many peaks rather than a constant level, this averaging technique favors detecting speech.
A transmit noise average and a receive noise average are also developed. The transmit noise average is representative of the noise level of the operating environment for the speakerphone. The receive noise average measures the noise level on the line from the far end party. The transmit noise average and the receive noise average are both developed by measuring the lowest level seen by the analog-to-digital converter. Since background noise is generally constant, the lowest level samples provide a reasonable estimate of the noise level.
The transmit noise average is used in various ways in the operation of the speakerphone of the '046 patent (the "'046 speakerphone"). When the speakerphone is in its idle state, an average of the audio informational signals picked up by the speakerphone microphone from its environment (the "transmit signal average") is compared to such noise average, and the exceeding by such signal average of such noise average by more than a certain threshold, is a factor which, among others, tends to induce switching from the idle to the transmit state. Similar exceeding by the transmit signal average of the transmit noise average by more than a threshold when the speakerphone is in, respectively, the transmit state and the receive state are factors which, among others, tend, respectively, to induce retaining of the speakerphone in the transmit state, and the switching of the speakerphone from the receive state to the transmit state.
In all of the above cases, however, the transmit noise average has no first order effect on the level of the voice sounds reproduced by the speaker element of the speakerphone. Consequently, with progressive increase in the average ambient noise level of the acoustic environment of that speaker element, the reproduced voice signals tend to become less and less intelligible.